Infections due to the Gram-negative bacteria Neisseria meningitidis and N. gonorrhoeae represent major public health problems around the world. Meningococcal infections total 0.5-1.2 million and cause death of 50,000-135,000 individuals annually worldwide. Infections by N. meningitidis can present with a range of symptoms including sudden onset of fever, headache, stiff neck, nausea, and alterations in mental state. The bacteria primarily infect the central nervous system causing cerebrospinal meningitis, but can also cause disseminated disease leading to an overwhelming inflammatory response known as sepsis that can result in vascular leakage, failure in multiple organs, and death or long term sequelae including amputation of limbs, deafness, and seizures. Declines in meningococcal disease have occurred in the last decade in many developed countries due in part to use of polysaccharide-protein conjugate vaccines to specific serogroups of N. meningitidis, although the vaccines are not 100% effective and do not provide protection against all serogroups or strains. In addition, two new vaccines for serogroup B disease have been approved recently, however approval is only for persons aged 10-25 years, and unanswered questions exist regarding efficacy. Paradoxically, N. meningitidis infects the nasopharynx of 8-20% of the population without causing disease. There are an estimated 106 million new cases of sexually transmitted gonococcal infections worldwide each year. Those most affected by the disease are women in whom infections are often asymptomatic. From 10- 20% of infected women suffer from pelvic inflammatory disease that can cause chronic pain, infertility, and ectopic pregnancy. Importantly, a growing number of studies have shown that gonococcal infection can facilitate the transmission of HIV. The lack of a vaccine and increasing number of strains of N. gonorrhoeae that are resistant to treatment with antibiotics have heightened the possibility of loss of control of the transmission of gonorrhea. Our work has focused on understanding the interactions of a major component of the bacterial membrane, the lipooligosaccharide (LOS), with the human immune system. Preformed components of what is termed the innate immune system are the first line of defense in protection against Gram-negative bacteria such as Neisseria. We have found that the ability of Neisseria to induce a feedback down-regulation of the innate immune response is positively correlated with the inflammatory potential of the bacteria which in turn is mediated by the expression of particular molecular substituents on the LOS. Our data also show that certain elements of the LOS structure assist the bacteria in evading lysis by the complement cascade in the blood and recognition by human antibodies that facilitate phagocytosis by white blood cells. These data are supported by the findings from several other laboratories in the field and previous results from our own research. This project is focused on testing and developing potential new therapeutic agents that will inhibit the bioactivity of the LOS by enzymatic removal of phosphate and acyl groups on the lipid and by inhibiting the genetic expression or competing for binding specifically to the active site of LOS biosynthetic enzymes. In addition, we will exten our observations regarding the inflammatory potential of the LOS and invasive ability of the Neisseria to interactions with an extensive set of relevant cell types that play a role in Neisseri infections. We will study models of infection in whole blood, in nasopharyngeal, cervical, and endothelial epithelial cells, monocytic cells, vascular endothelial cells, and brain cells, and analyze the bioactivity of LOS when inside of cells. We expect that the results will demonstrate the validity of new therapeutic targets, identify and develop potential new agents for the treatment of Neisseria infections, and increase our understanding of the complex relationship between these uniquely human pathogens and the innate immune system that enables the bacteria to cause disease.